There are a number of different methods of conducting handoff in a cellular communication system. Handoff is the act of transferring a call with a subscriber (e.g., mobile, portable, etc.) from one transceiver, a source transceiver, to a second transceiver, a target transceiver. In frequency division multiple access (FDMA) systems, such as the Analog Mobile Phone Standard (AMPS) system, when a call is handed from one transceiver to another, the connection to the first transceiver is broken and quickly established with a second transceiver. While handoffs can be performed quickly, the quality suffers during handoff and connections can be missed; resulting in calls being dropped.
To resolve these types of problems, the idea of soft and softer handoffs was developed. A soft handoff is a handoff between one site and another. A softer handoff is a handoff between one sector and another sector, both located at the same site. The basic concept of soft/softer handoff is that the new connection is established with the second transceiver before the connection with the first transceiver is broken.
The implementation of the present invention will be described in conjunction with direct sequence code division multiple access (DS-CDMA) systems, but is not limited to those applications. In DS-CDMA systems, the transmissions of a plurality of signals are carried over the same spectral resource (band width). This spectral resource may be either a wide (e.g., around 5 or more MHz) or narrow (e.g., around 1.2 or less MHz) frequency band. The signals are distributed, spread, throughout the band using a code. This same code is then used by the receiver to retrieve, despread, the signal.
The limitation on the number of signals which can be carried on the band is closely associated with the total power of all signals in the band. A more detailed description of power control in DS-CDMA systems is provided in U.S. patent applications: "Method for Compensating for Capacity Overload in a Spread Spectrum Communication System" having Ser. No. 07/783,751 filed on Oct. 28, 1991, now U.S. Pat. No. 5,245,627; and "A Method for Controlling Transmission Power in a Communication System" having Ser. No. 07/907,072 filed on Jul. 1, 1992, both assigned to the present assignee.
While the soft/softer handoff techniques provide higher quality signals to the receiver, it is at the expense of overall system capacity. When multiple transmitters are transmitting the same signal, capacity is being used that could be allocated to other users. Therefore, there is a need to reduce the amount of system power required to perform handoffs while at the same time retaining the benefits of soft/softer handoffs.